Distillation accounts for a significant amount of the overall energy usage in many industries, including crude oil refining and petrochemical production. Although over 40,000 distillation columns are used in a broad range of commercial applications, distillation in general is nevertheless characterized as having a low energy efficiency.
Distillation is a separation process that exploits differences (sometimes minor) in component relative volatilities or boiling points. Generally, a high degree of purity of component A (e.g., propane, having three carbon atoms) and component B (e.g., n-butane, having four carbon atoms) can be achieved by distilling an impure mixture of these components. This assumes that the distillation column used provides, in view of the relative volatility difference, a sufficient number of theoretical stages of vapor-liquid equilibrium contacting and that an azeotropic mixture of the components is not formed at a composition below the desired purity.
When separating a mixture of three components A, B, and C (or three fractions that may themselves be mixtures of components) at least two distillation columns are typically used; however, each column is only capable of separating a feed stream into two product streams, namely an overhead product enriched in the lower boiling component(s) and a bottoms product enriched in the higher boiling component(s).
An alternative to the use of two separate distillation columns for separating a mixture into three component streams is a dividing-wall column (DWC). A single DWC can replace the conventional two columns in series designs. Typically, the DWC has a lower energy consumption compared to the conventional two columns in series designs. An exemplary DWC is disclosed in U.S. Pat. No. 8,562,792, the entirety of which is incorporated herein by reference.
Despite being presumably effective for their intended purposes, a drawback of some DWC designs is the inability to effectively and efficiently control/split vapor flows across the two sides of the dividing wall. Current solutions to control/split vapor flows include adjusting the position of the dividing wall such that the required vapor flow is achieved on the two sides of the dividing wall by hydraulic pressure drop through the parallel paths on each side of the dividing wall. However, this arrangement poses a limitation, because it does not allow for the vapor flow to be adjusted whenever there is a substantial expected feed composition change or with any process objectives that results in a large, required vapor flow variation on either side of partition/dividing wall. Thus, since the pressure drop on either side of the walled sections is same, there are limitations in the application of DWCs—especially in cases where the flow up through each of the divided sections is must vary.
Therefore, it would be desirable to provide a DWC that does not suffer from these drawbacks and provides a process for separating hydrocarbons in which vapor passed to both sides of the dividing wall may be controlled and varied.